Coal pyrolysis is a process of thermochemical decomposition of coal in the absence of oxygen. The pyrolysis process produce gases, tar and char which are profitable raw materials and resources, as a case, gases and char can be used as fuel directly while tar can be hydro-treated to produce fuel oil. It is therefore considerable way of utilizing coal, especially for low rank coal. Researchers have put many efforts on the study of coal pyrolysis process since the early seventies of the last century during the worldwide oil crisis, such as Toscoal, Garrett, COED and CSIRO. Among them, the COED process shows high thermal efficiency, but the low yield of liquid products while Garrett yields more liquid products, but its post processing is difficult to run smoothly. In China, some new technologies have been proposed, such as, MRF, DG and so-called coal topping process developed by Kwauk . at IPE, which uses fast mixing downer as a pyrolyzer and coal ash as heat carrier, thus eliminating the need for sand or other solid carrier. Many research works show that the coal topping process is economically with higher yield of high value-added productions.
In our work, a model of coal pyrolysis in a downer reactor is built, which considers the hydrodynamic characteristics and the kinetics of coal pyrolysis together. The model aims to predict the yields of pyrolysis products under different conditions and the hydrodynamic characteristics inside the reactor such as the axial velocities of gases and solids, solids holdup and residence time of gases and solids along the downer. The model may provide valuable information on designing and operating the reactor.
2. Kinetic models of coal pyrolysis
A model of coal pyrolysis is built based on the work of Suuberg while experimental data used in the article are from Cui’s work . Considering it is easy to build and to get the products yields of coal pyrolysis compared with the other models, such as Anthony, by which only the yields of char, tar and total amount of gases could be obtained.
According simulation results, it can be found that the yield of solids decreases with the increasing of pyrolysis temperature due to further pyrolysis, which produces more volatiles, including liquid and gas products. For produced volatiles, total yields of gases increase continuously as temperature increases while yield of liquid (tar and water) increases until a maximum (about 20 wt% of the Hlh coal on dry basis) at about 600°C is reached, and then decreases with the increasing of temperature because high temperature makes tar cracking more than its formation.
3. Modeling of coal pyrolysis in a gas-solid cocurrent downer reactor
The coal topping process is based on the circulating fluidized bed (CFB) and a cocurrent downflow reactor is adopted as the pyrolyzer for its less solids resident time, more uniform gas-solid radial distribution and less back-mixing than riser reactor. In a downer type coal pyrolyzer, only solids, coal and ash, enter into the reactor from the inlet. With the proceeding of coal pyrolysis, the volatiles escape from the fractures of coal and move down cocurrently with the heated coal and ash. At last, volatiles and solids are fast separated by a cyclone.
4. Results and discussion
Based on the above model, simulations of the coal pyrolysis in a downer reactor were carried out using Hlh coal. It can be found that yields of liquid and gases increase fast near the inlet region and then almost keep unchanged along the downer because of a fast complete coal devolatilization while most of the volatiles, gases and liquid (tar and water), can be extracted from the fractures of the coal in a very short time (about 0.4s). As a result, the necessary height of the downer for the complete devolatilization is about 1.0m, further increase in the height of the downer does not change the yields significantly. As expected, higher temperature leads to more yield of gases due to further coal devolatilization. However, higher temperature also causes liquid products to crack. It suggests that the pyrolysis temperature should be controlled below 670°C if liquid product is preferred, above 860°C in the case more gas products are needed.
From the simulation of the axial distribution of volatiles (liquid and gases) and solids velocities along the downer, it can be found that for a fixed pyrolysis temperature, the velocity of volatiles shows a sharp increase near the inlet (about 1.0m) and then keep nearly constant, which demonstrates that the pyrolysis is nearly completed at about 1.0m of the downer at the same time. The increase of solids velocity is the combined result of the resultant force, drag force and gravity.It should be noted that the little solids (coal and ash) holdup will make the coal very difficult to maintain the required pyrolysis temperature and ensure a fast heat transfer conditions, especially for pilot or industrial scale reactor. It is a big challenge for the smooth operation of this process.
The model of coal pyrolysis in a downer reactor is built by combining Suuberg’s coal pyrolysis model with cocurrent downflow circulating fluidized bed model, in which both the hydrodynamic and the kinetic behaviors of coal pyrolysis are considered together. By this model, not only the yields of key products of coal pyrolysis can be predicted, but the hydrodynamic information inside the reactor, such as axial velocities of solids and gases, solids holdup, resident time of gases and solids along the downer can be obtained too.
A series of simulations using Huolinhe coal as feedstock are carried out at different temperatures. The simulation results show that residence time within 0.4s allow a nearly complete coal devolatilization near the inlet region at a fixed pyrolysis temperature of 600°C. Higher pyrolysis temperature improves the efficiency of coal pyrolysis. It increases the total yield of gases and liquid products. However, liquid product shows decreasing trends with the increasing of pyrolysis temperature especially when temperature is higher than 670°C due to the further crack of liquid product. It should be noted that the pyrolysis temperature should be controlled below 670°C if liquid product is preferred, otherwise, the higher temperature should be selected.
 Kwauk, M. (1998). Coal topping process. In Selected Papers of 9th Member Forum of Academia Sinica (pp. 202-204).
 Suuberg, E. M., Peters, W. A., & Howard, J. B. (1978). Product composition and kinetics of lignite pyrolysis. Industrial & Engineering Chemistry Process Design and Development, 17(1), 37-46.
 Cui, L. J.,Yao, J. Z., Lin,W.G.,&Zhang, Z. (2003a). Effects of temperature on products of flash pyrolysis of lignite in a spouted-entrained bed. Modern Chemical Industry,329 23(10), 28-32 (in Chinese).
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